Joint Ventures - Schedule of Condensed Financial Statements (Details) - USD ($) $ in Millions	Jan. 02, 2026	Oct. 03, 2025	Dec. 27, 2024	Sep. 27, 2024
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 247	$ 437
Total assets	11,198	11,460
Current liabilities	2,088	2,354
Total liabilities	6,536	6,840
Total Amentum equity	4,556	4,505
Non-controlling interests	106	115
Total shareholders' equity	4,662	4,620	$ 4,563	$ 4,552
Total liabilities and shareholders' equity	11,198	11,460
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Cash and cash equivalents	116	167
Current assets	192	191
Total assets	308	358
Current liabilities	119	146
Total liabilities	119	146
Total Amentum equity	132	153
Non-controlling interests	57	59
Total shareholders' equity	189	212
Total liabilities and shareholders' equity	$ 308	$ 358